The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,192,554; and in U.S. patent application Ser. No. 070,319, filed Aug. 27, 1979, by Chang Yul Cha and Thomas E. Ricketts, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS, now abandoned. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554 disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. A plurality of horizontally spaced apart vertical columnar explosive charges, i.e., an array of explosive charges, is placed in each of the unfragmented zones. The explosive charges are then detonated to explosively expand each unfragmented zone upwardly and/or downwardly toward the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the total volume of the voids. In effect, the volume of the excavated voids is distributed between the particles of the fragmented mass upon explosive expansion of the zones of unfragmented formation. The ratio of the volume of the void spaces between the particles to the total volume of the fragmented mass in the retort is the "void fraction" of the fragmented mass. The void fraction is generally given as a percentage and generally is between about 15% and 35%.
After the unfragmented formation is explosively expanded, retorting of the resulting fragmented mass is carried out to recover shale oil from the oil shale.
U.S. patent application Ser. No. 070,319 discloses a method for explosively expanding formation containing oil shale toward a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross-section similar to the horizontal cross-section of the retort being formed is initially excavated. A plurality of vertically spaced zones of unfragmented formation are left above the void. Explosive is placed in each of the unfragmented zones and detonated for explosively expanding such zones toward the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying zones can be expanded toward the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
It is desirable that a combustion zone passing downwardly through the fragmented permeable mass of formation particles in a vertical in situ oil shale retort be relatively flat. For example, when such a combustion zone is skewed (non-flat), shale oil and product gases produced in a lagging region of the combustion zone may be decomposed in a leading region of the combustion zone. Additionally, when the combustion zone is non-flat, the leading region of the combustion zone can reach the bottom of the retort before remaining regions. When any region of a combustion zone reaches the bottom of a retort, off-gas temperatures and/or the oxygen concentration in the off-gas can increase to above safe levels and it may be necessary to discontinue retorting. In this case, retorting is discontinued while oil shale particles in the fragmented mass downstream from lagging regions of the combustion zone remain unretorted.
It is, therefore, desirable to provide a method for explosively expanding oil shale formation toward a horizontal free face which results in the fragmented permeable mass of formation particles in the retort having a configuration which enhances the flatness of a combustion zone passing through it.